1. Field Of The Invention
This invention relates to the preparation of 1,1-difluoroethylene. More particularly this invention relates to a process for the preparation of 1,1-difluoroethylene by photochlorination of 1,1-difluoroethane with chlorine and pyrolysis of the reaction products thereof without isolation of chlorination by-products or the like.
2. Discussion Of The Prior Art
It is known to prepare 1,1-difluorethylene directly from 1,1-difluorethane. The difluorethane is reacted with chlorine at temperatures between 600 and 650.degree. C. Chlorosubstituted difluorethylenes are also formed as by-products (cf. U.S. Pat. No. 2,722,558). In this process the high temperature is extremely disadvantageous, since it causes the known metallic reactor materials to be attacked by the chlorine. For the production of 1,1-difluorethylene on a large technical scale, therefore, this process is unsuitable.
Furthermore, the 50 to 60% yields of 1,1-difluorethylene obtained in this method are entirely unsatisfactory. An increase in yield can be achieved by performing the reaction in the presence of dichlorodifluoromethane. However, even then the yields are still unsatisfactory (U.S. Pat. No. 2,723,296). This variant of the process, however, does not solve the problem of preventing the corrosion of the reactor material, so that this process does not eliminate the disadvantages of the process mentioned above.
To eliminate the disadvantages of these two known processes, a twostep process is described in German Offenlegungsschrift No. 21 45 975, in which 1,1-difluorethane is chlorinated in parallelly conducted process steps, and independently thereof, 1,1-difluor-1-chlorethane is pyrolytically cleaved to 1,1-difluorethylene. In this process it is expressly pointed out that the pyrolysis is to be performed with a pure 1,1-difluor-1-chlorethane from which the hydrogen chloride formed in the chlorination has been removed, as well as any excess chlorine that may be present.
This process has the disadvantage that two complete process steps are performed and only the working up (recovery) of the process products is performed in common. For the separation of the process products, however, a relatively great investment in apparatus is required. In addition, the yields of 1,1-difluoroethylene, which must be related only to the pyrolytic reaction, amount to less than 90%, which is unsatisfactory for operations on a large commercial scale.
The problem thus existed of preparing 1,1-difluorethylene from 1,1-difluorethane so as to obtain yields of more than 95%, with correspondingly high transformations. The process is furthermore to be performed in a single step, and requires only a small investment in apparatus. In addition, the problem existed of finding a process in the performance of which little or no corrosion is caused by chlorine to the reactor material.
It is an object of this invention, therefore, to provide a process for the preparation of 1,1-difluoroethylene from 1,1-difluoroethane whereby yields of 1,1-difluoroethylene in excess of 95% are realized.
It is a further object of this invention, therefore, to provide a simple process for the preparation of 1,1-difluoroethylene from 1,1-difluoroethane wherein large investments in apparatus are not required.
It is a further object of this invention to provide such a process whereby the manipulative procedures can be carried out continuously without the separation of intermediates.
These and other objects of this invention will become more apparent from the following description and claims.